[mlir][nfc] Update 2 tests for PadOpVectorizationWithTransferWritePattern (#122721)

* Relocates two tests for `PadOpVectorizationWithTransferWritePattern`
  in "vectorization-pad-patterns.mlir" to group them with other tests
  for the same pattern.
* Adds a note clarifying that these are negative tests and explains the
  reasoning behind them.
* Removes `transform.apply_patterns.linalg.decompose_pad` from the TD
  sequences as it's no longer needed (*).

This is essentially a small clean-up in preparation for upcoming
changes.

(*) `transform.apply_patterns.linalg.decompose_pad` was split off from
`transform.apply_patterns.linalg.pad_vectorization` in #117329.
"vectorization-pad-patterns.mlir" is meant to test the latter, not the
former.

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